From the 1p side, to activate hit diagonally up/left +1p + fire to activate, hit diagonally down/right +1p+fire to deactivate. I am not sure what it does besides make the counter bigger. (For the 2p side, do the same thing except hit 2p instead of 1p).
Violet and Hisa do the same thing in their replays.
Here is a link to the page where I found the trick:
http://gameplay.jp/~useless/videogames/ ... ta/04.html